1. Field of the Invention
The invention generally relates to data communication systems such as WLAN (Wireless Local Area Network) systems, and in particular to the correction of phase errors in signals received by data communications receivers in such systems.
2. Description of the Related Art
A Wireless Local Area Network is a flexible data communications system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN devices transmit and receive data over the air, minimizing the need of wired connections. Thus, WLAN systems combine interconnectivity with user mobility.
Most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communications systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to the 802.11b standard that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible as far as it relates to direct sequence spread spectrum technology, but it adopts a new modulation technique called CCK (Complementary Code Keying) which allows the speed increase.
In WLAN systems as well as in other spread spectrum communication systems, the signal on its way from the transmitter to the receiver experiences several distortions which may lead to a frequency error.
Assuming s(t) to be the transmitted signals(t)=A(t)·ejωtwhere ω is the carrier frequency, the received signal can be described asr(t)=B(t)·ej[(ω+ωe)t+φe(t)]where ωe is the carrier frequency difference between receiver and transmitter, and φe is the difference in phase between the receiver and the transmitter.
Turning now to FIG. 1, an error correction arrangement is schematically shown consisting of a frequency error correction unit 100 and a phase error correction unit 110. The frequency error correction unit 100 is used to compensate for the frequency difference, and the phase error correction unit 110 will then compensate for the residual phase error. This will now be described in more detail.
Assuming the baseband signal input to the frequency error correction unit 100 be given asB(t)·ej(ωet+φ0)the output signal of the frequency error correction unit 100 will beB(t)·ej({tilde over (ω)}et+φ0)This signal can be considered a signal with time dependent phaseφe(t)={tilde over (ω)}et+φ0which will linearly grow in time, as {tilde over (ω)}e and φ0 are constant values.
The phase error correction unit 110 has now the task to remove the remaining phase error such that the received signal is as close as possible to the transmitted signal, to minimize the probability of demodulation errors. An example of how the phase error correction unit 110 may operate is depicted in FIG. 2.
The phase error correction unit 110 of FIG. 2 includes an error correction module 200 that performs the following operation:B(t)·ejφe(t)·e−j{tilde over (φ)}e(t)=B(t)·ej[φe(t)]where {tilde over (φ)}e(t) is the current estimate of the phase error. The error correction module 200 is controlled by means of an error signal received from the measurement module 210. The measurement module 210 measures the phase error of the output signal of the correction module 200 and tries to generate the error signal so as to minimize the phase difference φe(t)−{tilde over (φ)}e(t).
However in conventional systems, the phase difference cannot be completely extinguished since the loop structure of FIG. 2 has a built in loop time delay so that the error correction module 200 will at any time receive an error signal that comes too late. This will be more apparent from FIG. 3.
As mentioned above, the output signal of the frequency error correction unit 100, i.e. the input to the phase error correction unit 110, will have a time dependent phase which linearly grows in time. This will lead to the sawtooth curve shown in FIG. 3 (noting that the turn-over limit of π/4 shown in the figure relates to QpsK modulation while e.g. in BpsK, this limit would be π/2). As the processing delay in the error measurement module 210 leads to a loop time delay T between the output signal of the correction module 200 and the error signal, the error signal {tilde over (φ)}e(t) will be delayed by this time. As apparent from FIG. 3, this leads to a residual phase error Δφ that may be substantially constant in time.
Thus, even if the error measurement module 210 will exactly measure the phase error, and even if the error correction module 200 will operate precisely, there will still be a residual phase error at the output of the conventional phase error correction unit due to the delay in the loop structure. Such loop time delay may also occur in phase error correction units of a different construction compared with that of FIG. 2, provided that there is a feedback structure in the arrangement. Moreover, the delay may have several delay components pertaining to any or all circuits found in the loop structure.
As there is still a residual phase error in the corrected signals, the demodulation reliability and throughput in subsequent signal processing units may be decreased.